A flow control servovalve can comprise a cylindrical housing, a controllable cross section flow duct or passage, a valve seat, and a valve member and at least one auxiliary valve mounted in the housing.
The valve seat can be formed as a mushroom-shaped member whose mushroom head has an edge forming a seating surface and whose stem is connected to at least one supporting cross piece extending across the flow duct.
The valve member can be a spring-loaded hollow piston guided in the housing whose interior space forms the flow duct which is mounted with its inner edge positionable on the seating surface of the mushroom head.
The auxiliary or pilot valve has a spring-loaded auxiliary stopper member with an associated auxiliary valve seat which controls a flowing medium branching from the main flow by a metering element.
In the valve of French Pat. No. 15 68 846 the mushroom member is pear shaped and is mounted with its pear neck acting as the mushroom stem on a rigid cross piece which extends practically over the entire length of the pear neck. This structure has a large inertia and is not elastically deformable. The cylindrical piston is guided with its outer edge over interposed sealing members in the associated cylindrical housing.
The metering or throttling elements comprise metering ducts and metering holes. In the described structure adhesion can occur between the mushroom member and the valve seat which is highly disturbing because the valve should be sensitive and operate reliably with the pressure drop developed through the return action of the cylindrical piston. The small forces involved however often are not sufficient, because of this adhesion, to release the tube like valve body from the closed position.